There are numerous systems which employ a cable or flexible tendon which is carried on or driven by pulleys or sheaves. In most of these systems the problem of derailment of the cable from the sheaves is a serious and perplexing one. There are usually numerous safeguards in structures which are employed to attempt to prevent derailment, but these apparatus are not always foolproof.
One system in which derailment of a cable from its carrier sheaves is particularly troublesome and dangergous is in a chairlift of the type employed to transport skiers. Most chairlifts involve several thousand feet of cable stretched over periodically spaced pulleys mounted on towers. Some chairlifts are in excess of one mile in length, and constant supervision and observation of the movement of the chair-carrying cable over the pulleys along the entire length of the lift is normally not possible. Moreover, the method of attaching the chairs to the cable inherently prevents the use of cable derailment prevention structures on one side of the cable.
Occasionally, the cable for a chairlift may become derailed from one or more of the pulleys on which it is mounted. Usually the cable will be caught by a supporting structure so that danger to the passengers is minimized. If the chairlift is not stopped almost immediately after derailment, however, there is some danger that the derailed cable will further be pulled from the supporting structure or entangled in the same. If this should happen, there is a substantial hazard, particularly in steep mountainous terrain.
In order to increase the safety of chairlifts, attempts have been made to sense the derailment of the cables so that the lift can be immediately shut down and the cable replaced on the sheaves. One approach to this problem is to position a frangible electrically conductive element proximate the cable in the area in which the cable will move upon derailment. The cable will strike the frangible element upon derailment, and the frangible element can be used as a switch to automatically shut down the operation of the lift and indicate where the derailment has taken place. One such system is described in my prior U.S. Pat. No. 3,822,369 entitled "Frangible, Flexible Printed Circuit Sensor Fracturable by Derailed Cable," issued July 2, 1974. Another type of frangible cable sensing device is shown in U.S. Pat. No. 3,230,327.
While the use of frangible switches in connection with sensing cable derailment has generally been found to be quite satisfactory, there is always the theoretical possibility that a cable will derail in a manner causing the same to jump over the frangible switch. Accordingly, other approaches have been employed in attempt to sense cable derailments in chairlift systems.
In most chairlifts, each tower includes a series of sheaves or pulleys which are mounted in pairs to yokes, which in turn is mounted to a frame on the end of the tower arm. Each yoke is usually pivotally mounted to the frame, and accordingly, cable derailment systems have been employed in which one of the sheaves or pulleys on a yoke is formed from aluminum while the other is formed from cast iron. When the cable derails, the cast iron pulley outweighs the aluminum pulley and will be gravity biased downwardly and actuate a switch mounted proximate the yoke. This system, however, has been found to have some serious drawbacks. The chairlifts are often subjected to very adverse environmental conditions. Thus, it is not uncommon for a yoke to be frozen against pivotal movement, preventing actuations of the derailment sensing switch. Moreover, derailment sensing switches have been found to malfunction as a result of their exposure to vibrations on the pulley frame and the extreme adverse weather conditions. Another approach is based upon the pivotal mounting of the frame carrying the sets of pulleys and yokes to the end of the tower arm. A wire or tendon is connected at one end to the frame and at the other end to a tension spring which is secured to the tower cross arm. A sensor is mounted to the tower cross arm and senses movement of the tendon or wire. When the cable is derailed from the pulleys, the spring causes the frame to rotate about the end of the cross arm, and the resulting movement of the wire or tendon can be sensed to shut down the chairlift. This system has also been found to have certain defects. First, the moment of inertia of the frame carrying anywhere between four and ten or more pulleys is very substantial. Moreover, the moment arm employed in such devices does not provide the desired mechanical advantage. Accordingly, the high moment of enertial, poor mechanical advantage and icing can combine to produce a system which is unreliable or intolerably slow to react to a cable derailment. In addition, this system again exposes the derailment sensing switch to highly adverse operating conditions.
Accordingly, it is an object of the present invention to provide a positive cable derailment sensing apparatus and method which has improved reliability and safety in the sensing of derailment of a cable from pulley means.
Another object of the present invention is to provide a positive cable derailment sensing apparatus in which the sensing switch can be positioned in a remote, protected location so as to minimize the possibility of malfunctioning.
Another object of the present invention is to provide a positive cable derailment sensing apparatus in which a single switch can be used to monitor cable derailment on either side of a loop-type endless chairlift cable.
Still a further object of the present invention is to provide a positive cable derailment sensing apparatus which is fatigue and vibration resistant, is easy and inexpensive to manufacture, is easy to install, can be adapted to a wide variety of chairlift tower attitudes and configurations, and requires minimum maintenance.
The positive cable derailment sensing apparatus and method of the present invention has other objects and features of advantage, some of which will be set forth in more detail or will become apparent from the description hereinafter and the accompanying drawing.